You will often read that indoor air is two to five times more polluted than outdoor air. The line traces back to EPA's Total Exposure Assessment Methodology (TEAM) studies of the 1980s, which measured volatile organic compounds and often found them at higher levels indoors. It was true for those chemicals in those homes. It is not a rule for every pollutant, and EPA's own pages now say indoor levels can "sometimes" be higher.
What holds up
- We are mostly indoors. EPA's National Human Activity Pattern Survey (2001) found Americans spend about 90% of their time inside. Whatever is in your home air is most of what you breathe.
- Some pollutants are mainly indoor. Radon, cooking smoke, chemical vapors from products and furniture, and moisture that feeds mold all come from inside the house.
- Outdoor pollution gets in. During wildfire smoke, indoor particle levels often reach a third to two thirds of outdoor levels unless windows stay shut and a filter runs.
What is overstated
- "2 to 5 times worse" as a blanket claim. On a bad outdoor smog or smoke day, a closed home with a purifier is usually cleaner than outside.
- Gas stoves. A 2022 study estimated that about 12.7% of US childhood asthma could be attributed to gas stoves. That is a modeled estimate built on earlier studies, and it is debated. Cooking with gas does raise nitrogen dioxide indoors, so using a range hood that vents outside is sensible either way.
- CO2 and thinking. One small 2016 study found lower decision-making scores at higher CO2; later studies are mixed. See our CO2 monitor guide.
What you can control
- Test for radon. It is the indoor pollutant with the clearest harm. How to test.
- Vent what you make. Kitchen hood when cooking, bathroom fan when showering.
- Keep humidity at 30-50% to limit mold and dust mites.
- Filter particles with a HEPA purifier or a MERV 13 HVAC filter, especially on smoke days.
- Measure with an air quality monitor rather than guessing.
For the outdoor side, see air quality and wildfire smoke by state and county.